The simplest method for transmitting data between communication apparatuses is directly transmitting data from one communication apparatus to another communication apparatus. However, this method uses large transmission power to guarantee quality of the wireless communication, for example, when wireless communication is performed between a BS (base station) and a MS (mobile station), a result of which is the power consumption increasing. Alternatively, the size of a wireless cell may be reduced. However the number of base stations BS will increase and ultimately increase the system construction cost.
Accordingly, relay stations are provided between communication apparatuses. Hereinafter, a description will be given for a relaying method employed when a relay station (RS) is provided between a BS and a MS and two-hop transmission is carried out between the base station (BS) and the mobile station (MS).
FIG. 11A is a diagram illustrating an example of a relaying method. This relaying method uses the following four phases to bidirectionally transmit data between the BS and the MS. More specifically, in the first phase (P-1), the BS transmits a signal conveying data to the RS. In the second phase (P-2), the RS transmits the signal to the MS. In the third phase (P-3), the MS transmits a signal conveying data to the RS. In the fourth phase (P-4), the RS transmits the signal to the BS. In this relaying method, four communication resources having orthogonal relation are employed. This relaying method is a kind of “DF (Decode-and-Forward) relaying”, requiring four phases.
FIG. 11B is a diagram illustrating an example of a relaying method. This relaying method requires the following three phases to bidirectionally transmit signals between the BS and the MS. More specifically, in the first phase (P-1), the BS transmits a signal S1 to the RS via a link D1. In the second phase (P-2), the MS transmits a signal S2 to the RS via a link D2. In the third phase (P-3), the RS decodes the signals S1 and S2, calculates an exclusive OR (XOR) of each respective pair of bits of decoded data to combine them. After that the RS generates symbols corresponding to a modulation method based on the combined data and multicasts the calculated results to the BS and the MS. This relaying method is a kind of “DF (Decode-and-Forward) relaying” but requiring three phases.
In the DF relaying, three orthogonal communication resources are employed. Logically, throughput of two-hop communication is improved by 33% at the maximum in comparison with the relaying method illustrated in FIG. 11A and the throughput may approach the doubled value as the number of hops increases.
FIG. 12 illustrates a configuration of a network coding section for combining pieces of information received by the RS via a plurality of communication lines and for transmitting the combined result. As illustrated in the drawings, this network coding section includes an XOR unit 1, a header adding unit 2, a CRC (Cyclic Redundancy Check) unit 3, an FEC (Forward Error Correction) unit 4, and a modulating unit 5 (see, for example, R. Ahlswede, N. Cai, S. Y. R. Li and R. W. Yeung, “Network information flow”, IEEE Transactions on Information Theory, Vol. 46, No. 4, pp 1204-1216, July 2000).
The XOR unit 1 calculates an XOR of each respective pair of bits of pieces of decoded data, namely, a pair of bit strings, to generate multicast data. The header adding unit 2 adds a header to the output of the XOR unit 1. The CRC unit 3 adds a CRC bit, for error detection at the receiving side, to the output of the header adding unit 2. The FEC unit 4 performs error correcting coding, such as a turbo encoding, on the output of the CRC unit 3. The modulating unit 5 modulates a symbol corresponding to a modulation method based on the output of the FEC unit 4.
In the DF relaying, the RS performs the XOR calculation on each respective pair of bits (bit by bit calculation) to generate multicast data. Accordingly, the BS can acquire data transmitted from the MS by performing an XOR calculation of the multicast data and the data that the BS has transmitted to the RS. Similarly, the MS can acquire data transmitted from the BS by performing an XOR calculation of the multicast data and the data that the MS has transmitted to the RS. In this manner, bidirectional communication may be realized. Meanwhile, the DF relaying is described in “Coded bi-directional relaying” written by P. Larsson, N. Johansson, K. E. Sunell, the 5th Scandinavian WS on Wireless Ad-Hoc Networks(AdHoc '05), Stockholm, Sweden, May 2005.
FIG. 11C is a diagram illustrating another relaying method according to the related art. In this relaying method, signals are bidirectionally transmitted between the BS and the MS with the following two phases. More specifically, in the first phase, signal transmission from the BS to the RS and signal transmission from the MS to the RS are simultaneously carried out. In the second phase, the RS amplifies a signal (signal including interference of each other), which is a signal obtained by combining the signals having been simultaneously transmitted from the BS and the MS in a space, and multicasts the amplified signal to the BS and the MS. At this time, the RS does not decode the received signals for transmission. This relaying method is a kind of AF (Amplify-and-Forward) relaying.
The AF relaying employs two orthogonal communication resources. Logically, throughput of the AF relaying may become equal to a double of the throughput of the relaying method illustrated in FIG. 11A at the maximum. Meanwhile, the AF relaying is described in “Wireless network coding by amplify-and-forward for bi-directional traffic flows” written by P. Popovsiki, and H. Yomo, IEEE Communications Letters, Vol. 11, No. 1, pp 16-18, January 2007.
However, the relaying methods illustrated in FIG. 11A-11C have characteristics summarized in FIG. 13. Since each phase is independently controlled in the conventional relaying method, both of reliability of links and classical flexibility (degree-of-freedom) of communication are high. However, since the number of phases increases in this relaying method, the throughput reduces, as a result of which communication efficiency decreases. Here, “the flexibility of communication” means a degree of freedom for selecting a data transmission modulation scheme and a code rate, i.e., modulation and coding scheme (MCS).
In the DF relaying explained above, pieces of data are received from two links. After two bit strings having the same number of bits resulting from decoding of the received pieces of data are combined by calculating an XOR for two bit strings (bit by bit) to generate one bit string, modulation is performed based on symbols having certain number of bits corresponding to a certain modulation method by dividing this bit string. Accordingly, one modulation scheme is selected at the time of multicast of the bit string D3 to the BS and the MS. For example, even though up to 16QAM (16 Quadrature Amplification Modulation) is available for transmission from the RS to the BS and up to QPSK (Quadrature Phase Shift Keying) is available for transmission from the RS to the MS, the QPSK is selected for both the transmissions to satisfy transmission requirements of both the transmissions.
More specifically, when communication qualities of a pair of links are unbalanced, data transmission via a link having the preferable communication quality is restricted by data communication via a link having the unpreferable communication quality. That is, the flexibility of communication decreases.
The AF relaying explained above does not have the flexibility of communication. Additionally, since the noises are also amplified in the RS in an environment with many noises, communication performance significantly drops. Although introduction of, for example, a DNF (Denoise-and-Forward) relaying method reduces the seriousness of this problem, the introduction of the DNF relaying method also complicates a configuration of the RS.